Optical waveguide device and manufacturing method therefor

ABSTRACT

A manufacturing method for an optical waveguide device. The manufacturing method includes the steps of forming an optical waveguide in a substrate having an electro-optic effect, forming an SiO 2  film on the substrate, forming Si films on the SiO 2  film, the lower surface of the substrate, and at least a part of the side surface of the substrate to thereby make a conduction between the Si film formed on the SiO 2  film and the Si film formed on the lower surface of the substrate. The manufacturing method further includes the steps of applying a photoresist to the Si film formed on the SiO 2  film, patterning the photoresist so that a portion of the photoresist corresponding to the optical waveguide is left, forming a groove on the substrate along the optical waveguide by reactive ion etching, and removing the photoresist and the Si films.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical waveguide device and amanufacturing method therefor.

2. Description of the Related Art

An optical device using an optical waveguide has increased in necessitywith the evolution of optical communication, and it is used as anoptical modulator, optical demultiplexers, optical switch, or opticalwavelength converter, for example. Known examples of the opticalwaveguide include an optical waveguide formed by diffusing titanium (Ti)in a LiNbO₃ crystal substrate, an optical waveguide formed by depositingSiO₂ on an silicon (Si) Si substrate, and a polymer optical waveguide.As a practical external modulator, a Mach-Zehnder type optical modulator(LN modulator) using a dielectric crystal substrate such as a lithiumniobate (LiNbO₃) crystal substrate has been developed. Carrier lighthaving a constant intensity from a light source is supplied to the LNmodulator to obtain an optical signal intensity-modulated by a switchingoperation using the interference of light.

The LN modulator includes a dielectric substrate formed from a Z-cutlithium niobate crystal, a pair of optical waveguides formed in theupper surface of the substrate by thermally diffusing titanium (Ti) inthe substrate to thereby increase a refractive index, these opticalwaveguides being combined together near their opposite ends, an SiO₂buffer layer formed on each optical waveguide, and a signal electrode(traveling wave electrode) and a grounding electrode formed on thebuffer layers so as to respectively correspond to these opticalwaveguides. Signal light input from one end of the combined opticalwaveguides is split at one junction thereof to propagate in the opticalwaveguides. When a drive voltage is applied to the signal electrodeformed over one of the optical waveguides, a phase difference isproduced between the split signal lights propagating in the opticalwaveguides by an electro-optic effect.

In the LN modulator, these signal lights are recombined to be taken outas optical signal outputs. By applying the drive voltage so that thephase difference between the signal lights propagating in the twooptical waveguides becomes 0 or π, an on/off pulse signal can beobtained. As a recent LN modulator, the development of a modulatorhaving a high-frequency band of 40 Gb/s has been pursued to realize ahigher modulation rate. To reduce a propagation loss and ensure ahigh-frequency band characteristic in the above-mentioned high-frequencyband, it is indispensable to form a groove having a depth of severalmicrometers between the electrodes along the optical waveguides in theLN modulator. This groove is formed usually by using an RIE (reactiveion etching) dry etching device.

The conventional technique of forming the groove by using the RIE dryetching device has the following problems.

(1) The LN substrate as the base of the LN modulator is a ferroelectricmember, so that polarization due to temperature fluctuations occurs andwhen an electric field on one surface of the substrate reaches about6,000 V, discharging occurs to cause the damage to a wafer due todischarge shock. In particular, when a sudden temperature change (5°C./min or more) occurs, the damage to the wafer becomes remarkable. TheRIE device for use in forming the groove as mentioned above employs ahigh-frequency power supply, which causes a sudden temperature change tothe wafer. As a result, the damage to the wafer easily occurs to cause areduction in yield.

(2) The RIE device is divided into a load lock chamber (loading chamber)for setting the wafer or taking it out and an etching chamber foractually performing RIE. The wafer is automatically transferred betweenthe load lock chamber and the etching chamber. However, since the LNwafer exhibits a pyroelectric effect, it tends to stick to a metallicmember. In a conventional manufacturing method for an LN modulator, theLN wafer sticks to a stage (aluminum electrode) provided in the etchingchamber after etching of the LN wafer, so that the wafer cannot beautomatically transferred to the load lock chamber.

Accordingly, every time the etching step is ended, the etching chamberis disassembled and a sharp member such as a razor blade is insertedbetween the wafer and the stage to forcibly separate the wafer from thestage. However, this wafer separating work may easily cause the damageto the wafer, thus remarkably reducing the yield. Further, the etchingchamber disassembling work includes a dangerous operation of opening abreaker for the high-frequency power supply and manually disconnecting asignal line. Further, a cooling water pipe, gas induction pipe, etc.must be disconnected, causing a danger and trouble. Further, inreassembling the etching chamber, it is necessary to ensure the assemblyaccuracy of a wafer chucker, and even if there is a fine positioningerror, gas leak occurs to result in generation of a temperaturedistribution in the LN wafer, thus leading to the damage to the wafer.

(3) In the RIE dry etching, a photoresist is used as a mask, and the LNwafer with the photoresist is etched. At a high temperature (120° C. orhigher), the photoresist is erosively burned and oxidized. To preventthis problem, the wafer is cooled through the stage and maintained at alow temperature. Accordingly, a temperature difference is producedbetween the upper and lower surfaces of the wafer, thus leading to thedamage to the wafer.

(4) In the step of patterning exposure of the photoresist for the RIEdry etching, the wafer and a glass mask are aligned. This alignment mustbe performed with accuracy of 2 μm or less, so as to ensure necessarycharacteristics. However, since the LN wafer itself is transparent, theluminance at the time of exposure lacks and an alignment marker cannottherefore be viewed, causing a pattern deviation. If the patterndeviation arises, the photoresist must be applied and patterned again,causing a reduction in nonadjusted ratio and a reduction in yield.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide amanufacturing method for an optical waveguide device which can preventthe damage to the wafer in the RIE dry etching step and allows automatictransfer of the wafer.

It is another object of the present invention to provide an opticalwaveguide device having an excellent high-frequency characteristic.

In accordance with an aspect of the present invention, there is provideda manufacturing method for an optical waveguide device, including thesteps of forming an optical waveguide in a substrate having anelectro-optic effect; forming an SiO₂ film on the substrate; forming Sifilms on the SiO₂ film, the lower surface of the substrate, and at leasta part of the side surface of the substrate to thereby make a conductionbetween the Si film formed on the SiO₂ film and the Si film formed onthe lower surface of the substrate; applying a photoresist to the Sifilm formed on the SiO₂ film; patterning the photoresist so that aportion of the photoresist corresponding to the optical waveguide isleft; forming a groove on the substrate along the optical waveguide byreactive ion etching; and removing the photoresist and the Si films.

Preferably, the substrate includes a LiNbO₃ substrate, and the step offorming the optical waveguide includes the step of thermally diffusingTi in the LiNbO₃ substrate. For example, the step of forming the Sifilms is performed by sputtering. Preferably, the photoresist includes aconductive photoresist.

In accordance with another aspect of the present invention, there isprovided a manufacturing method for an optical waveguide device,including the steps of forming an optical waveguide in a substratehaving an electro-optic effect; forming an SiO₂ film on the substrate;forming Ti films on the SiO₂ film, the lower surface of the substrate,and at least a part of the side surface of the substrate to thereby makea conduction between the Ti film formed on the SiO₂ film and the Ti filmformed on the lower surface of the substrate; applying a photoresist tothe Ti film formed on the SiO₂ film; patterning the photoresist so thata portion of the photoresist corresponding to the optical waveguide isleft; forming a groove on the substrate along the optical waveguide byreactive ion etching; and removing the photoresist and the Ti films.

In accordance with a further aspect of the present invention, there isprovided an optical waveguide device including a substrate having anelectro-optic effect; an optical waveguide formed in the substrate; asignal electrode formed in relation to the optical waveguide; agrounding electrode formed on the substrate; a groove formed on thesubstrate along the optical waveguide; an SiO₂ buffer layer formed onthe substrate except the groove; and an Si film formed on the SiO₂buffer layer, the inner surface of the groove, the lower surface of thesubstrate, and at least a part of the side surface of the substrate.

In accordance with a still further aspect of the present invention,there is provided an optical modulator including a substrate having anelectro-optic effect; an optical waveguide structure having an inputwaveguide formed in the substrate, an output waveguide formed in thesubstrate, and first and second waveguides extending between the inputwaveguide and the output waveguide, the first and second waveguidesbeing connected to the input and output waveguides, respectively; asignal electrode formed over the first waveguide; a first groundingelectrode formed over the second waveguide; a second grounding electrodeformed over the substrate at a position opposite to the first groundingelectrode with respect to the signal electrode; a first groove formed onthe substrate along the first waveguide; a second groove formed on thesubstrate along the second waveguide; an SiO₂ buffer layer formed on thesubstrate except the first and second grooves; and an Si film formed onthe SiO₂ buffer layer, the inner surfaces of the first and secondgrooves, the lower surface of the substrate, and at least a part of theside surface of the substrate.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an optical modulator according to a preferredembodiment of the present invention;

FIG. 2 is a cross section taken along the line II—II in FIG. 1;

FIGS. 3A to 3N and 3P to 3U are sectional views for illustrating amanufacturing method according to a preferred embodiment of the presentinvention;

FIG. 4 is a schematic view showing the configuration of an RIE dryetching device;

FIG. 5A is a plan view showing a condition where a pair of protectivemembers are bonded to a wafer;

FIG. 5B is a cross section taken along the line 5B—5B in FIG. 5A; and

FIG. 6 is a plan view showing a dicing step.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a plan view of a Mach-Zehnder type optical modulator 2manufactured by the manufacturing method according to the presentinvention. FIG. 2 is a schematic cross section taken along the lineII—II in FIG. 1, wherein the dimensional ratios are exaggerated forillustration. The optical modulator or optical modulator chip 2 isformed of a dielectric having an electro-optic effect. For example, theoptical modulator 2 is formed from a lithium niobate substrate (LiNbO₃substrate) 4. The optical modulator 2 has a Mach-Zehnder type opticalwaveguide structure 6. The optical waveguide structure 6 is composed ofan input optical waveguide 8, an output optical waveguide 10, and firstand second optical waveguides 12 and 14 extending between the inputoptical waveguide 8 and the output optical waveguide 10.

The first and second optical waveguides 12 and 14 are connected througha Y branch 16 to the input optical waveguide 8 and also connectedthrough a Y branch 18 to the output optical waveguide 10. The opticalwaveguide structure 6 is formed by thermally diffusing titanium (Ti) inthe LiNbO₃ substrate 4. Signal light supplied to the input opticalwaveguide 8 is substantially equally divided in optical power into twocomponents, and these two components are guided by the first and secondoptical waveguides 12 and 14, respectively. These guided opticalcomponents are coupled through the Y branch 18 to the output opticalwaveguide 10. Switching is made between a coupled mode where light isguided in the output optical waveguide 10 and a radiation mode (leakymode) where light is radiated from the Y branch 18 into the substrate 4according to a phase difference of light guided in the first and secondoptical waveguides 12 and 14.

A signal electrode (traveling wave electrode) 20 is provided over thefirst optical waveguide 12 and a grounding electrode 22 is provided overthe second optical waveguide 14, so as to change the phase betweensignal lights branched. Further, another grounding electrode 24 isformed on the substrate 4 opposite to the grounding electrode 22 withrespect to the first optical waveguide 12. A groove 26 having a depth ofseveral micrometers is formed on the substrate 4 between the signalelectrode 20 and the grounding electrode 24 so as to extend along thefirst optical waveguide 12. Similarly, a groove 28 having a depth ofseveral micrometers is formed on the substrate 4 between the signalelectrode 20 and the grounding electrode 22 so as to extend along thesecond optical waveguide 14. These grooves 26 and 28 are formed for thepurposes of reducing a propagation loss and ensuring a high-frequencyband characteristic.

A sectional structure of the optical modulator 2 will now be describedwith reference to FIG. 2. An SiO₂ buffer layer 30 is formed on thesubstrate 4 except the grooves 26 and 28. The entire surface of thesubstrate 4 including the inner surfaces of the grooves 26 and 28 iscovered with an Si film 32. A Ti film 34 is formed on the Si film 32 byvacuum evaporation, and an Au film 36 is formed on the Ti film 34 byvacuum evaporation. Further, an Au plating 38 is formed on the Au film36. The signal electrode 20 and the grounding electrodes 22 and 24 areformed by the Au plating 38. Each of these electrodes 20, 22, and 24 hasa thickness of about 30 μm.

There will now be described a fabrication process for an opticalmodulator as an example of the manufacturing method for the opticalwaveguide device according to the present invention. As shown in FIG.3A, a Ti film 40 having a thickness of about 100 nm is formed by vacuumevaporation on a LiNbO₃ substrate (LN substrate) 4. The Ti film 40 has apurity of 99.99%. Next, a photoresist 42 having a thickness of about 1μm is applied to the Ti film 40, and then patterned as shown in FIG. 3B.Next, the Ti film 40 is etched by wet etching with the photoresist 42used as a mask. After ending this etching, the photoresist 42 is removedby ultrasonic cleaning with acetone or the like, thereby obtaining apattern of the Ti film 40 as shown in FIG. 3C.

In the next step, thermal diffusion of the Ti film 40 into the substrate4 is performed at a temperature of about 1000° C. for about 10 hours aspassing pure oxygen as a carrier gas at a flow rate of about 10liters/min, thereby forming optical waveguides 12 and 14 in the uppersurface of the substrate 4 as shown in FIG. 3D. As shown in FIG. 3E, anSiO₂ buffer layer 30 having a thickness of about 1 μm is next formed. Asshown in FIG. 3F, Si films 44 a, 44 b, and 44 c each having a thicknessof about 0.1 μm are formed on the buffer layer 30, the lower surface ofthe substrate 4, and the opposite side surfaces of the substrate 4,respectively. The deposition of these Si films 44 a, 44 b, and 44 c isperformed by a DC sputter device using Ar as a carrier gas at adeposition pressure of 0.66 Pa.

While the Si film 44 c is formed on the opposite side surfaces of thesubstrate 4 in this preferred embodiment, it is sufficient that the Sifilm 44 c may be formed on at least a part of either side surface of thesubstrate 4 so as to make a conduction between the Si film 44 a formedon the SiO₂ buffer layer 30 and the Si film 44 b formed on the lowersurface of the substrate 4. Since the upper and lower surfaces of thesubstrate 4 are covered with the Si films 44 a and 44 b, and these Sifilms 44 a and 44 b are connected with each other through the Si film 44c formed on the opposite side surfaces of the substrate 4, it ispossible to suppress polarization of the LN substrate 4 due totemperature fluctuations, thereby preventing charging of the LNsubstrate 4 at a high voltage.

In the next step, a photoresist 46 is applied to the Si film 44 a andthereafter patterned for RIE dry etching as shown in FIG. 3G. Then, RIE(reactive ion etching) is performed by using an RIE dry etching deviceshown in FIG. 4 to thereby form a pair of grooves 26 and 28 each havinga depth of several micrometers and extending along the opticalwaveguides 12 and 14 (FIG. 3H). As shown in FIG. 3I, the photoresist 46and the Si films 44 a, 44 b, and 44 c are removed by using a removingliquid (e.g., DE-3, a trade name manufactured by Tokyo Ohka Kogyo Co.,Ltd.).

The RIE dry etching device used herein will now be described withreference to FIG. 4. The RIE dry etching device has an etching chamber50 and a loading chamber (load lock chamber) (not shown), wherein an LNwafer 51 is automatically transferred between the etching chamber 50 andthe loading chamber by means of a transfer device. Reference numeral 52denotes a stage formed of aluminum. The stage 52 is connected to a biashigh-frequency power supply 62 and functions as an electrode. The stage52 is formed with a plurality of holes 54 serving both as He gas supplyholes and as insertion holes for hoist pins 56. Reference numeral 58denotes a high-frequency power supply for forming a plasma. Thehigh-frequency power supply 58 is connected to a high-frequency antenna60. Reference numerals 64 and 66 denote a magnetically neutral linearcoil and a magnetically neutral discharging portion, respectively. Theetching chamber 50 is connected to a vacuum pump 68, so that the etchingchamber 50 is maintained under a given vacuum by the vacuum pump 68during RIE.

The operation of this RIE device will now be described. First, the wafer51 is set on a pan for a wafer handler in the loading chamber. When anoperation control button is depressed, the wafer 51 is automaticallytransferred from the loading chamber to the etching chamber 50 by thetransfer device. In the etching chamber 50, the wafer 51 placed on thestage 52 is raised by the hoist pins 56 operated to project upward fromthe lower side of the stage 52, and the pan for the wafer handler isreturned to the loading chamber by the transfer device. Thereafter, thehoist pins 56 are lowered to set the wafer 51 on the stage 52.

A mixed gas of Ar and C₃F₈ as an etching gas is supplied from a supplypipe 70 into the etching chamber 50, and an antenna power of 1200 W fromthe high-frequency power supply 58 and a bias power of 200 W from thebias power supply 62 are applied under a deposition pressure of 0.25 Pato perform the reactive ion etching (RIE) of the wafer 51. The wafer 51or the substrate 4 in this preferred embodiment has such a structurethat the Si film 44 a formed on the upper surface of the substrate 4 andthe Si film 44 b formed on the lower surface of the substrate 4 areconnected with each other through the Si film 44 c formed on theopposite side surfaces of the substrate 4 as shown in FIG. 3G.Accordingly, polarization of the wafer 51 or the substrate 4 can beprevented to thereby prevent charging of the wafer 51 or the substrate4.

As a result, discharging from the wafer 51 can be prevented to therebyprevent the damage to the wafer 51 due to discharge shock. Further,since charging of the wafer 51 can be prevented, sticking of the wafer51 to the stage 52 can be eliminated to thereby allow automatic transferof the wafer 51. Further, since the Si film 44 a is formed under thephotoresist 46 in patterning exposure for dry etching, a sufficientluminance can be ensured to allow clear viewing of a marker formed onthe wafer 51, so that alignment can be easily made in the patterningexposure.

After ending the etching, the wafer 51 is raised by the hoist pins 56,and next received by the pan for the wafer handler to automaticallytransfer the wafer 51 to the loading chamber by the transfer device.Thereafter, cleaning of the etching chamber 50 is performed. Theinterior of the etching chamber 50 is contaminated with the photoresist46 removed from the wafer 51. Unless cleaning of the etching chamber 50is performed, high-frequency discharge cannot be effected in the nextcycle of the RIE operation, so that RIE cannot be performed.

The cleaning is performed in the following manner. A dummy wafer such asan Si wafer is placed on the wafer pan in the loading chamber, and thisdummy wafer is automatically transferred to the etching chamber 50.Then, the cleaning is performed for a given time period in an atmosphereof oxygen gas containing a small amount of CF₄ under a vacuum of about1.33 Pa at an antenna power of 1000 W and a bias power of 50 W. Afterending the cleaning, the dummy wafer is automatically transferred to theloading chamber.

After ending the step shown in FIG. 3I, an Si film 32 having a thicknessof about 0.1 μm is formed on the upper surface, the lower surface, andat least a part of the opposite side surfaces of the substrate 4 by a DCsputter device as shown in FIG. 3J. Thereafter, a Ti film 34 having athickness of about 50 nm and an Au film 36 having a thickness of about200 nm are sequentially formed by vacuum evaporation under a vacuum of6.6×10⁻⁴ Pa as shown in FIG. 3K. The Ti film 34 has a purity of 99.99%,and the Au film 36 has a purity of 99.99% or more.

Thereafter, a photoresist 72 is applied with a thickness of about 13 μmon the Au film 36, and then patterned as shown in FIG. 3L. Next, the Tifilm 34 and the Au film 36 are etched by using an etching liquid asshown in FIG. 3M. After this etching, the photoresist 72 is removed byultrasonic cleaning with acetone or the like (FIG. 3N). Thereafter, aphotoresist 74 for an Au plating is applied with a thickness of about 32μm, and then patterned as shown in FIG. 3P.

Thereafter, an Au plating 38 having a thickness of about 30 μm is formedas shown in FIG. 3Q, and the photoresist 74 is next removed byultrasonic cleaning with acetone or the like (FIG. 3R). Thereafter, aphotoresist 76 for etching of an unwanted portion of the Ti film 34 andthe Au film 36 is applied and next patterned as shown in FIG. 3S. Then,the unwanted portion of the Ti film 34 and the Au film 36 is removed bywet etching as shown in FIG. 3T, and the photoresist 76 is next removedby ultrasonic cleaning with acetone or the like (FIG. 3U). As a result,given electrode shapes 20, 22, and 24 can be obtained.

The above steps shown in FIGS. 3A to 3U are carried out to thereby allowthe formation of a plurality of optical modulators 2 on the LN wafer 51.In the next step, a pair of protective members (auxiliary appliances) 80are bonded to the LN wafer 51 at positions near the opposite ends of theplural optical modulators 2 formed on the LN wafer 51 as shown in FIGS.5A and 5B. These protective members 80 serve to protect the end surfacesof each optical modulator 2.

Thereafter, dicing by a rotary resin diamond blade is performed toindividually cut the optical modulator chips 2 from the LN wafer 51 asshown in FIG. 6. An Si film is formed on the side surfaces of eachoptical modulator chip 2 to electrically connect the Si films 32 formedon the upper and lower surfaces of the substrate 4 in the step of FIG.3J. Finally, an antireflection film is formed by vacuum evaporation onthe end surfaces of each optical modulator chip 2, thus completing eachoptical modulator chip 2.

While the Si films 44 a, 44 b, and 44 c are formed on the upper, lower,and side surfaces of the substrate 4, respectively, in the step of FIG.3F in this preferred embodiment, the Si films 44 a, 44 b, and 44 c maybe replaced by Ti deposited films formed by vacuum evaporation. Further,a conductive photoresist is preferably used as the photoresist 46 in thestep of FIG. 3G, so as to prevent charging of the upper surface of thesubstrate 4 or the wafer 51.

While the manufacturing method according to the present invention hasbeen applied to a manufacturing method for an optical modulator in thispreferred embodiment, the present invention is not limited to the abovepreferred embodiment, but it is also applicable to a manufacturingmethod for any other optical waveguide devices such as an opticaldemultiplexer, optical switch, and optical wavelength converter.

According to the manufacturing method for the optical waveguide deviceof the present invention, the Si films are formed on the entire uppersurface, the entire lower surface, and at least a part of the sidesurface of the LN wafer prior to the dry etching (RIE) step.Accordingly, charging of the LN wafer can be prevented to therebyeliminate the damage to the wafer due to discharging at a high voltage,thus improving the manufacturing yield. Further, since charging of theLN wafer can be prevented, sticking of the wafer to the stage in the RIEstep can be eliminated to thereby allow automatic transfer of the wafer.

Moreover, it is unnecessary to disassemble the etching chamber andthereafter reassemble it, so that the working efficiency can be greatlyimproved. Further, since the Si film is formed under the photoresist inpatterning exposure for dry etching, a sufficient luminance can beensured to allow clear viewing of a marker formed on the wafer, so thatalignment can be easily made in the patterning exposure.

The present invention is not limited to the details of the abovedescribed preferred embodiments. The scope of the invention is definedby the appended claims and all changes and modifications as fall withinthe equivalence of the scope of the claims are therefore to be embracedby the invention.

1. A manufacturing method for an optical waveguide device, comprising:forming an optical waveguide in a substrate having an electro-opticeffect, said substrate having upper, lower, and side surfaces; formingan SiO₂ film on said substrate; forming silicon (Si) films on said SiO₂film, the lower surface of said substrate, and at least a part of theside surface of said substrate to thereby make a conduction between saidSi films formed on said SiO₂ film and said Si films formed on the lowersurface of said substrate; applying a photoresist to said Si filmsformed on said SiO₂ film; patterning said photoresist so that a portionof said photoresist corresponding to said optical waveguide is leftattached on said Si films; forming a groove on said substrate along saidoptical waveguide by reactive ion etching; and removing said photoresistand said Si films.
 2. The manufacturing method according to claim 1,wherein said substrate comprises a LiNbO₃ substrate, and forming saidoptical waveguide comprises thermally diffusing titanium (Ti) in saidLiNbO₃ substrate.
 3. The manufacturing method according to claim 1,wherein said forming said Si films is performed by sputtering.
 4. Themanufacturing method according to claim 1, wherein said photoresistcomprises a conductive photoresist.